1. Field of the Invention
The present invention relates to a picking device for a workpiece by using a robot manipulator, in particular a picking device and a picking method for reducing the time required for identifying an object by means of a vision sensor.
2. Description of the Related Art
In an application for picking an object or a workpiece from “a random pile”, in which a plurality of workpieces are not positioned orderly on a pallet or the like, but are stacked disorderly in a box or the like, a camera is used for capturing an image of the workpieces in a wide range, in order to roughly find the position of the workpiece to be picked. For example, U.S. Pat. No. 5,987,591 discloses a sensor robot system combining a CCD camera for widely imaging workpieces and a laser sensor for measuring the three-dimensional positions of the workpieces. It is undesirable if the workpieces cannot be detected or captured in the image by the camera due to obstacles positioned in the field of view of the camera. Therefore, a picking device such as a robot manipulator must be moved outside the camera's field of view when capturing an image of the workpieces. However, this may increase the cycle time of the whole system that includes such picking devices.
When the workpieces are somewhat positioned orderly, once an image of the workpieces is obtained, the workpieces may be detected without capturing a new image of the workpieces every time a workpiece is picked up. Such a technique is known. However, when the workpieces are piled disorderly as in the above “random pile”, frequently the position and/or the orientation of one workpiece is changed by picking another workpiece. Accordingly, while picking the workpiece from a “random pile”, only when the picking device such as a robot manipulator moves the picked workpiece to a certain place outside of the camera's field of view, the image of another workpiece is captured. For example, Japanese Unexamined Patent Publication No. 7-319525, discloses a picking device capable of detecting a workpiece in a two-dimensional image captured by a camera, which matches a verifying model among prerecorded verifying models, in order to speed-up the picking operation from a random pile.
In the above technique, a newly captured image is required after a workpiece is detected and picked. Therefore, when a workpiece is successfully picked, the next image may be captured without delay, during the transportation of the workpiece. However, if the picking operation fails, in order to detect a next workpiece, it is necessary to move the picking device out of the field of view of the camera so as to capture a wide-range image, and/or to move the robot manipulator such that the area to be detected is within the field of view of the camera, and as a result, the system cycle time may increase.